Automatic temperature regulating apparatus



Patented Apr. 1, 1947 AUTOMATIC TEMPERATURE REGULATING APPARATUS Samuel G. Eskin, Los Angeles, Calif., and Charles K. Strobel, Pittsburgh, Pa., assignors to Robertshaw Thermostat Company, Youngwood, Pa., a corporation of Pennsylvania.

Application April 13, 1944, Serial No. 530,902

12 Claims. 1

This invention relates to automatic temperature regulation and, more particularly, to apparatus for the purpoze employin safety control.

One object of this invention is to regulate a variable, such as temperature conditions, by means effective for safety control also of the fuel burning device which causes the conditions.

Another object of the invention is to bring the safety and condition controlling system into operation immediately upon turning on the fuel supply for the burner.

Another object of the invention is to determine automatically whether fuel flow to the burner shall continue after ignition thereof should have occurred.

Another object of the invention is to insure positive operation of a fuel flow controlling member to a position to admit fuel to the fuel burner while rendering it fully responsive to the safety control for operation to fuel flow preventing position.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawing, wherein is shown a schematic view of a safety control apparatus for gaseous fuel burners em bodying this invention.

Referring more particularly to the drawing, a gaseous fuel burner I is supplied with fuel by a main fuel pipe l2. The fuel supp y is controlled by a main valve M which, in this instance, is arranged for manual operation by a suitable knob or handle l6. Interposed in the fuel supply pipe l2 between the main valve l4 and the burner II] is an electromagnetic or solenoid gas valve l8 of any known tyne which is arranged to be opened by energization of the solenoid and closed by gravity upon deenergization thereof. As will be apparent hereinafter, the o ening of the solenoid valve to admit fuel to the main burner I 0 is contingent upon current of a predetermined minimum valu being supplied there to. A pilot burner 29 is su plied with fuel by a conduit 22 which receives fuel from the main fuel pine l2 when the main va ve I4 is in open position a d which is not under control of the solenoid valve I8.

Electric current for the operation of th apparatus is su plied by the 1ie wires 24 connected to the prirrary of a tra sformer 26 which may receive commercial current at the primary at a voltage var ation from 105 to 125 volts. In this embodiment, the main valve M has a main switch combined therewith which is connected to one line wire 24 by wires 28 and 30. Any suitable means may be incorporated in the combined valve and switch structure to close the switch portion when the main valve portion thereof is: actuated to open position. The secondary of the transformer 26 will deliver the current from the line wires 24 at Voltages varying in accordance with the primary voltage variation above and below an average of six volts.

Connected to one terminal of the transformer secondary by wires 3|, 32 and 33 is one junction A of a first parallel circuit mesh consisting of a thermal resistance element 34 in one branch and the solenoid valve l8 and a thermal cut-out 36 in the other branch thereof. The thermal resistance element 34 is positioned adjacent the pilot burner 20 where it is exposed to the heat of the flame thereof and will vary appreciably in its resistance according to its heated or unheated condition. The element 34 may be in the form of a coil of wire formed of material having normally a relatively low resistance and a positive temperature coeflicient of resistance forming a ballast resistor. It will be apparent, however, that other types of flame sensitive, heat responsive or thermal resistance elements having the required characteristics may be substituted and that it is not essential that the flame of the pilot burner 20 actually impinge on the coil.

The opposite junction B of this parallel mesh is connected by wires 31 and 38 with one junction 0 of a second parallel circuit mesh included in the secondary circuit. The opposite junction D of the second parallel mesh is connected by wire 6| to the other terminal of the transformer secondary 26. One branch of this second parallel mesh cons sts of a resistance element 40, hereinafter designated as a valve control resistor, also preferably in the form of a coil of wire having normally a relatively low resistance and a positive temperature coefficient of resistance forming a ballast resistor, although a constant resistor could be employed. The resistor 40 is not. however, exposed to the heat of any flame but is adapted to become heated upon passage of electric current therethrough and will change its resistance appreciably in accordance with changes of temperature caused by changes in the value of current flowing through it. The other branch of the second parallel mesh consists of a constant resistance element 42, hereinafter designated as a pick-up resistor for the solenoid valve [8, and a control switch 44 connected in series circuit therewith. The control switch 44 is adapted to respond to a variable caused by operation of the main burner l0 and may take the form of a thermostatic switching means such as a temperature regulator of the snap-acting type. As ind cated in broken lines in the drawing, the knob or handle l6 for operating the combined valve and switch l4 could be utilized for setting the temperature regulator to operate at a desired temperature.

The temperature regulator 44 may comprise a pair of levers 4648 connected by an over-center spring 50 and supported at opposite ends for snap ac ion in kni e-ed ed bearings carried on a pivoted frame 52. -The lever 48 is adapted to be engaged by the actuating portion of a thermally sensitive element 54, the other portions comprising the usual capillary tube 55 and bulb 56 which is located in the space to be heated by the main burner In. It will be apparent that this space could well be an oven of a domestic range, the temperature of which will be regulated by operation of the thermostat in accordance with the setting of the handle IS. The lever 46 carries, at the end opposite the knife-edge bearing therefor, a contact 58 adapted for movement into and out of engagement with a stationary contact 60 to open and close the branch of the parallel mesh in which the regulator is located. It will be apparent that the snap-acting thermostatic regulator 44 disclosed herein is in schematic form only and various modifications could be made in the details of construction and operation for commercial use. The opposite junction D of this second parallel mesh is connected by a wire 6| to the other terminal of the transformer secondary 2B.

In addition to the shunt established on the solenoidvalve I8 by the flame sensitive element 34. this valve is also arranged to be short-circuited for a predetermined period of time. The temporary short-circuit is established by a control switch having a timing element 62 which is initially closed and embodies a predetermined time delay period. The tim ng element 82 is connected, as will be hereinafter apparent, as a third branch of the first parallel mesh. The timing element 82 forms one of a pair of such devices of the mercury tube type, the other one of which. designated by the numeral 64, serves a purpose to be described hereinafter. Both timing element 62-84 are shown diagrammatically as comprising the usual glass tubes containing globules of mercury which, in the initial closed position of the elements, bridge electrode pairs 66-68 and 10-12, respectively, located at one end of the switches. Opera ion of the elements 8264 from the initial position shown in the drawing to a tilted or open position may be performed through the interposition of a bell-crank lever 14 carrying the timing elements on one arm thereof and adapted to be moved by engagement of the opposite arm thereof with a cam element 16 carried by the handle l6.

When the timing elements are thus tilted by the lever 14, the mercury unbridges the electrodes but only after a predetermined period fixed by the time it requires for the mercury to flow through restricted orifices l8 and 80 into reservoirs at the opposite ends of the elements. As indicated schematically in the drawing, the timing element 82 has a shorter time-delayed action in opening than does the timing element 64. In this embodiment, the timing element 62 opens after a time delay of approximately six seconds, while the timing element 64 opens after a time delay of approximately eighteen seconds. Both elements, however, close substantially instantaneously as may be accomplished by providing a relatively unobstructed path for flow of the mercury around the orifices 18-80 from the reservoirs back to the electrodes. It is to be understood that this is merely a diagrammatic representation and that the control switch need not necessarily be of the mercury type or, if of 'such type need not be tilting, as long as the delayed opening and instantaneous closing features are incorporated.

The longer delay timing element 64 is adapted to control the energization of an electrical igniting means 82 for the pilot burner 20, which igniting means may be in the form of a coil of wire adapted to reach incandescence upon passage of current therethrough for a predetermined period. Preferably, a constant resistance element 84 is included with the ignitor 82 in the circuit controlled by the longer time delay element 64 in order to afford proper balancing of the circuit in respect to resistance values. As will be herein after described the long delay timing element 64, the igniter 82 and the resistance element 84 are connected as a third branch of the second parallel mesh.

In order to bring the system into normal running operation the main valve I4 is manually opened and will permit fuel to fiow to the pilot burner 20. Although such manual operation of the main valve I4 will serve to close the main switch incorporated therewith, the solenoid valve 18 is short-circuted by the timing element 82 and is not energized at this time so that no fuel can fiow to the main burner Ill. The closing of the main switch of valve [4 does, however, energize the secondary circuit consisting of the transformer secondary 26, the first described parallel mesh and the second described parallel mesh, all connected in series as follows: from one terminal of the transformer secondary 26, wire 3|, wire 32, wire 33, junction A of the first parallel mesh which consists of thermal resistance element 34 in one branch and solenoid valve I8 and cut-out 36 in the other branch, junction B of this first parallel mesh, wire 31, wire 38, junction C of the second parallel mesh which consists of regulator contacts 58 and 60 and pick-up resistor 42 in one branch and valve control resistor 40 in the other branch, junction D of this second parallel mesh, and wire 6| back to the other terminal of the transformer secondary 26. Moreover, the cam element 16 serves to tilt the lever 14 upon operation of the handle it to open main valve I4 causing the timing elements 62 and 64 to be tilted from the initial closed position shown in the drawing to their open position for tim delayed action. It may be assumed that the oven or other space to be heated by the main burner Ill is as yet unheated and the temperature regulator 44 has its contacts 58 and 68 engaged, as will occur under such conditions.

The igniter 82 is energized through a circuit which may be traced from the transformer secondary 26, wire 3|, wire 32, wire 33, junction 'A of the first parallel mesh, wire 88, electrodes 68 and 68 of the short delay timing element 62, common wire 88, electrodes 18 and 12 of the long delay timing element 84, wire 98, igniter 82, wire 92, constant resistor 84, wire 94 and wire 6| back to th secondary of the transformer 28. The fuel flowing from the pilot burner 28 is ignited by the igniter 82 as soon as the coil thereof reaches igniting temperature. The heat of the flame at the pilot burner 28 serves to increase the temperature of the thermal resistance element 34 which thereupon increases in resistance, as previously described. At the end of approximately six seconds, which is the time it takes for timing element 62 to undergo its delayed opening, the short circuit established by this element on the solenoid valve I8 is removed and the circuit is as follows: from one terminal of the transformer secondary 26, wire 3|, wire 32, wire 33, junction A of the first parallel mesh which consists of thermal resistance element 34 in one branch and solenoid valve l8 and cut-out 38 in the other branch, junction B of this first parallel mesh, wire 3?, wire 38, junction C of the second parallel mesh which consists of regulator contacts 58 and 68 and pick-up resistor 42 in one branch and valve control resistor 48 in the other branch, junction D of this second parallel mesh, and wire 6| back to the other terminal of the transformer secondary 26. This circuit is, of course, the same as that existing before the timing element 82 opened. The only change occurs in the igniter circuit which is now as follows: from the transformer secondary 26, wires 3|, 32 and 33, junction A of the first parallel mesh, which consists of thermal resistance element 34 in one branch and solenoid valve 18 and cut-out 38 in the other branch, junction B of this first parallel mesh, wire 31, wire 38, common wire 88, electrodes 18 and 12 of the long delay timing element 64, wire 98, igniter 82, wire 82, constant resistor 84, wire 94 and wire 6! back to the secondary of the transformer 28. The secondary current values are so adjusted that when this short-circuit is removed sufficient current flows through the operating coil of the solenoid valve l8 to cause opening of this valve regardless of whether or not the thermal resistance element 34 is heated by the pilot burner flame. Such action may be termed :a forced pick-up for the solenoid valve.

The energizing of the coil of the solenoid valve l8 sufilciently to cause forced pick-up and opening of this valve occurs during the second delay period, that is, between the time of opening of the timing element 62 and opening of the timing element 34 which period is between six and eighteen seconds. In this period, the igniter current is somewhat reduced because of the increase in secondary circuit impedance caused by the removal of the short circuit across the first parallel mesh containing the solenoid valve l8. As this removal occurs while the igniter 82 is still connected in the second parallel mesh and is thus establishing therein a relatively low impedance, it is apparent that the required value of current may be caused to flow through the coil of the solenoid valve it to cause it to open as described even if the thermal resistance element 34 is un-- heated by the pilot burner flame and remains of relatively low resistance value.

At the end of the second delay period, that is, at the expiration of eighteen seconds following opening of the timing element 62, the igniter 82 and the constant resistance element 84 are removed from the circuit by opening of the timing element 64. The circuit then consists of the first and second parallel meshes connected in series circuit with the transformer secondary 2 B. This circuit may be traced from the transformer secondary 26, wire 3|, wire 32. wire 33, junction A of the first parallel mesh which consists of the thermal resistance element 34 in one branch and the solenoid valve I 8 and thermal cut-out 35 in the other branch, junction B of this first parallel mesh, wire 31, wire 38, junction C of the second parallel mesh which consists of the regulator contacts 58 and 88, pick-up resistor 42 in one branch and valve control resistor 48 in the other branch, junction D of this second parallel mesh, and wire 61 back to the secondary of the transformer 28. By the opening of the timing element 64 the total secondary impedance is again increased and the total secondary current is decreased. Consequently, the current through the coil of the solenoid valve I8 is also decreased but the valve may remain open as will now be described.

If, at the end of the eighteen second time delay period, the thermal resistance element 34 has been sufliciently heated by the pilot burner flame, its resistance has been increased sufficiently to cause current of a predetermined minimum value to be applied to the coil of the solenoid valve l8. The value of this current is sufiicient to maintain this valve in the open position and also to cause pick-up from closed position as will be apparent hereinafter. However, if due to ignition or other failure, the thermal resistance element 34 has not been sufiiciently heated, there is insufllcient current through the coil of the solenoid valve 18 to maintain this valve in the open position when the igniter and resistance element 84 are deenergized. It is thus apparent that the determination Whether the fuel continues to flow through the main burner l8 after the igniter 82 is removed from the circuit depends upon whether the pilot burner 28 has previously become ignited and produced a flame to heat the thermal element 34.

Moreover, if at any time during the operation of the apparatus, the fiame of the pilot burner 28 becomes extinguished, then the thermal resistance element 34 will cool and its resistance be lowered sufiiciently to render the current through the coil of the solenoid valve l8 insufiicient to retain the valve in open position.

Since, under the circumstances of cooling of thermal res stance element 34, the current to the coil of the solenoid valve I8 must become lowered from above pick-up value to below release value, the closing time of this valve would ordinarily be unduly prolonged. However, the valve control resistor 48 is made sensitive to current changes in the circuit in order to accelerate the closing of the solenoid valve I 8 whenever an increase in the resistance of the resistor 48 occurs. Thus, whenever an increase in the current through the resistor 48 takes place, as it does when the thermal resistance element 34 cools and its resistance becomes reduced, the resulting temperature rise of the resistor 48 causes an increase in voltage drop across this element and a rapid decrease in voltage drop across the coil of the solenoid valve I8 to accelerate closing action.

Assuming that the thermal element 34 is heated and the solenoid valve remains open, the oven or other space temperature will eventually reach the value for which the regulator 44 has been set. The contacts 58 and 68 are then opened by operation of the regulator to cause removal of the pick-up resistor 42 from the circuit. Such deenergization of the pick-up resistor 42 causes an increase in the total secondary impedance so that the current through the coil of the solenoid valve I8 is decreased sufficiently to cause the valve to close while the thermal resistance element 34 still remains heated. The secondary circuit under these conditions consists of the transformer secondary 26, the first parallel mesh of the solenoid valve l8, thermal cut-out 3B and thermal resistance element 34 and the second parallel mesh which now consists of one branch only containing the resistor 48, all connected in series circuit.

amass.)

As the circuit connections for these elements remain unchanged from those previously traced, further description is unnecessary.

When the temperature which is being regulated by the regulator 44 decreases so that the contacts 58-60 are again closed then suflicient current flows once more through the coil of the solenoid valve l8 to cause this valve to open as lon as the thermal resistance element 34 remains heated by the flame of the pilot burner 20. If, however, the pilot burner flame has been extinguished before the closing of the contacts 58-60. or the thermal resistance element 34 is otherwise unheated, then the closing of the contacts 5860 cannot cause current of pick-up value to flow to the coilof the solenoid valve l8. Thus, after the initial starting period of eighteen seconds, the pick-up resistor 42 under control of the regulator 44 serves to cause opening and closing of the solenoid valve l8 when the thermal resistance element 34 is heated by the pilot burner flame, whereas when the thermal resistance element '34 is not heated and is relatively cool, the current through the coil of the solenoid valve 18 is insuflicient to either open the valve or maintain it in the open position. It is thus apparent that during the normal runnin period of the burner the fuel supply is contingent upon current of a predetermined minimum value being supplied to the coil of the solenoid valve or otherwise this valve will close and shut off the fuel supply to the main burner l0. 7

Considerable flexibility of circuit arrangement i provided by this invention as the value of the respective resistances can be varied to suit different operating conditions. The single solenoid valve functions as a condition and safety control in the arrangement described and a complete safety and condition controlling system has been provided. As the solenoid valve is actuated to open position under forced pick-up conditions at the end of six seconds, no chattering of the valve should result. The time delay and instantaneous reset control switch is disclosed as being manually operated as distinguished from electrically operated devices which introduce complications into the circuits in which they are used. It will be apparent that various changes may be made in the form and arrangement of parts and in the details of construction herein disclosed within the scope of the appended claims without departing from the spirit of the invention.

We claim:

1. In apparatus of the character described having main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a source of current supply for said device, thermal resistance means connected to said source and said device and exposed to the heat of a flame at the pilot burner for varying in resistance and spective of the heated condition of said thermal resistance means. g

2. In apparatus of the character described hav-' ing main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a source of current supply for said device, a thermal resistance element having a preselected temperature coeflicient of resistance and normally a relatively low resistance, said element being connected to said source and said device and exposed to the heat of a flame at the pilot burner for increasing in resistance in response to said flame and causing an increase in the current value to said device, resistance means cooperablewith said element for increasing the total circuit resistance and causing said current value to fall below said minimum. means for rendering said resistance means eifective after a predetermined period if said element has not become heated, and means responsive to a variable caused by operation of the main burner for rendering said resistance means effective after said period irrespective of the heated condition of said element.

3. In apparatus of the character described having main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a source of current supply for said device, a thermal resistance element connected to said source and exposed to the heat of a flame at the pilot burner for varying in resistance and causing variations in the current value to said device, resistance means cooperable with said element for varying the total circuit resistance and causing said current value to fall below said minimum, means for establishing a shunt for said resistance means, means for rendering said resistance means effective after a predetermined period if said element has not become heated, and switching means responsive to a variable caused by operation of the main burner for discontinuing the shunt for said resistance means and rendering the latter effective after said period irrespective of the heated condition of said element.

4. In apparatus of the character described having main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a source of current supply for said device, a thermal resistance element in shunt with said device and exposed to the heat of a flame at the pilot burner for varying in resistance and causing variations in the current value to said device, resistance means in series circuit with said device and cooperable with said element for varying the total circuit resistance and causing said current value to fall below said minimum, means for establishing a shunt for said resistance means, means for rendering said resistance means effective after a predetermined period if said element has not become heated, and switchin means responsive to a variable caused by operation of the main burner for discontinuing the shunt for said resistance means and rendering the latter effective after said period irrespective of the heated condition of said element.

5. In apparatus of the character described having main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a source of current supply for said device, an electric igniter for the pilot burner, means for deenerglzing said igniter after a predetermined period during which said pilot burner is adapted to produce a flame, thermal resistance means exposed to the heat of said flame for varying in resistance and causing variations in the current value to said device in response to said flame. means cooperable with said thermal resistance means for varying the total circuit resistance and causing said current value to fall below said rninimum upon said igniter deenerg zation if said thermal resistance means has not become heated. and means responsive to a variable caused by operation of the ma n burner for rendering the last said means effective after said period irrespective of the heated condition of said thermal resistance means.

6. In apparatus of the character de cribed having main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner. said fuel supply being contingent upon curre t of a predetermined minimum value being supplied to said device. a source of current supply for said device an electric igniter for the pilot burner. means for deenergizing said igniter after a predetermined period during which said pilot burner is adapted to produce a flame, a thermal resistance element connected to said source and ex osed to the heat of said flame for varying in resistance and causing variations in the current value to said dev ce in response thereto, resistance means cooperable with said element for varying the total circuit resistance and causing said current value to fall below said minimum upon said igniter deenergization if. said element has not become heated, means for establishing a shunt for said resistance means, and means responsive to a variable caused by operation of the main burner for discontinuing the shunt for said resistance means and rendering the latter effective a ter said period irrespective of the heated condition of said element.

'7. In apparatus of the character described having main and pilot burners, the combination of an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a source of current supply for said device, an electric i gniter for the pilot burner adapted to reach igniting temperature after a predetermined period of energization, means for maintaining said current value below said minimum for said period and thereafter being ineffective to prevent said fuel supply, means for deenergizing said igniter after a predetermined period of energization thereof additional to the first said period and during which said pilot burner is adapted to produce a flame, thermal resistance means exposed to the heat of said fiame for varying in resi tance and causing variations in the current value to said device in response to said flame, means cooperable with said thermal resistance means for varying the total circuit resistance and causing said current value to fall below said minimum upon said igniter deenergization if said thermal resistance means has not become heated, and means responsive to a variable caused by operation of the main burner for rendering the last said means eflective after said period irrespective of the heated condition of said thermal resistance means.

8. In apparatus of the character described having main and pilot burners, the combination of an electrically operable device movable between positions for controlling the supply of fuel to the main burner, an lectric igniter for said pilot burner connected in series circuit with said device, a control-switch having a timing element short-circuiting said device for a predetermined period during which said igniter may reach igniting temperature, a thermal resistance element connected in parallel circuit with said device and exposed to the heat of the flame at the pilot burner, a second resistance element connected in series circuit with said device and forming a parallel mesh with said igniter, a third resistance element connected in shunt with said second element, a second control switch having a timing element for deenergizing said igniter after a predetermined period additional to the first said period and causing movement of said device to prevent said fuel supply unless said thermal resistance is heated, and a thermostatic switch responsive to thermal conditions caused by operation of the main burner for deenergizing said third resistance element after said additional period to cause said movement irrespective of the heated condition of said thermal resistance.

9. A control system for fuel burners having main and pilot burners, comprising an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a circuit for supplying current to said device of at least said minimum value, a thermal resistor in said circuit and located in proximity of the pilot burner flame for varying in resistance value according to the presence or absence of said flame, means including a timing element electrically connected to said resistor and cooperable therewith for varying the total circuit resistanc after a predetermined period during which said resistor has time to vary in resistance value, said current value thereby being reduced below said minimum unless said resistor has varied, and a regulator operatively responsive to a condition caused by operation of the main burner for adjusting the resistance value of the last said means after said period and causing said current value to be reduced below said minimum irrespective of whether said resistor has varied.

10. A control system for fuel burners having main and pilot burners, comprising an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent upon current of a predetermined minimum value being supplied to said device, a circuit for supplying current of at least said minimum value to said device, a thermal resistor in said circuit having a positive temperature coefilcient of resistance and located in proximity of the pilot burner flame for varying in resistance value according to the presence or absence of said flame, means including a timing element electrically connected to said resistor and cooperable therewith for varying the total circuit resistance after a predetermined period during which said resistor has time to become heated, said current value thereby being reduced below said minimum unless said resistor is heated, and a regulator operatively responsive to a condition caused by operation of the main burner for ad- 11 justing the resistance value of the last said means after said period and causing said current value to be reduced below said minimum while said resistor remains heated.

11. A control system for fuel burners having main and pilot burners, comprising an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent on current of a predetermined minimum value being supplied to said device, a circuit for supplying current to said device of at least said minimum value, a thermal resistor in said circuit and located in proximity of the pilot burner flame for varying in resistance value according to the presence or absence of said flame, a plurality of resistance means in circuit with said resistor, said resistance means being arranged to adjust the total circuit resistance so that current of said minimum value is supplied to said device, switching means including a timing element operative for electrically disconnecting certain of said resistance means after a predetermined period during which said resistor has time to become heated, said operation causing an increase in said total circuit resistance sufiicient to reduce said current value below said minimum unless said resistor is heated, and a regulator operatively responsive to a condition caused by operation of the main burner for electrically disconnecting other of said resistance means after said period, said regulator operation causing an increase in said total circuit resistance sufiicient to reduce said current value below said minimum while said resistor remains heated.

12. A control system for fuel burners having main and pilot burners, comprising an electrically operable device for controlling supply of fuel to the main burner, said fuel supply being contingent on current of a predetermined minimum 12 value being supplied to said device, a circuit for supplying current of at least said minimum value to said device, a thermal resistor in said circuit and located in proximity of the pilot burner flame for varying in resistance value according to the presence or absence of said flame, an electric resistance igniter for the pilot burner in circuit with resistor, a plurality of resistance means in circuit with said resistor, said igniter and resistance means being arranged to adjust the total circuit resistance so that current of said minimum value is supplied to said device, switching means including a timing element operative for electrically disconnecting said igniter after a predetermined period during which said resistor has time to become heated, said operation causing an increase in said total circuit resistance sufllcient to reduce said current value below said minimum unless said resistor is heated, and a regulator operatively responsive to a condition caused by operation of the main burner for electrically disconnecting certain of said resistance means after said period, said regulator operation causing an increase in said total circuit resistance sufiicient to reduce said current value below said minimum while said resistor remains heated.

SAMUEL G. ESKIN.

CHARLES K. STROBEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,287,248 Holmes June 23, 1942 1,535 240 Norris Apr. 28, 1935 2,237,578 Ray Apr. 8, 1941 

